This invention relates to a structure and method of fabricating a temperature sensitive conductive element, and more particularly to a structure and method of fabricating a thermistor for very large scale integrated (VLSI) circuits.
The successful miniaturization of integrated circuits (ICs) has been a primary enabler of fast evolving computer technology. Solid state processing techniques have also enabled a continual advancement in the miniaturization of electronic devices that are included in these ICs. In a miniaturized environment, however, careful monitoring of all physical factors is needed because even the slightest variation of these factors can detrimentally affect the performance of the IC. To enable careful monitoring, sensing devices also known as “sensors” have been developed and incorporated into most ICs. Sensors are themselves electronic devices that are designed selectively to measure wide varieties of physical quantities including force, acceleration, pressure, chemical concentration and temperature. The sensors are also miniaturized but most often they are fabricated on a size scale similar to that of current very large scale integration (VLSI) devices.
Temperature is one of the more important physical factors that need careful monitoring. External exposure to certain temperatures can harm devices, but also internal temperature variations can be symptomatic of other serious design or operational problems existing elsewhere in the IC. In miniaturized devices, temperature control becomes even more important because of the reduced size of the device that makes it more susceptible even to the slightest temperature variations.
In addition to the situations already enumerated, temperature sensing is especially useful in ICs using VLSI devices. Such circuits depend on temperature sensing for autonomic processor control. A locally monitored temperature excursion can trigger current reroute with the slightest temperature change outside the acceptable range before damage occurs such as in a particular transistor or circuit block. Similarly, integrated temperature sensor circuits could be used to locally and dynamically adjust supply voltages to particular transistors and circuit blocks to minimize the temperature differences across the chip and safe guard against such damage and to improve performance and timing. The performance of a particular transistor might be reduced by local adjustment of supply voltages. However, the overall chip or system performance and timing is optimized because transistors across the chip are better matched.
Consequently, temperature monitoring is extremely crucial and useful in the operation of a range of IC circuits. Temperature sensors have been introduced to address this need.
Temperature sensors, or temperature-sensing devices, are varied and include a range of devices such as thermocouples, reversed-biased diodes, and temperature-sensitive resistors. Each of these temperature-sensing devices has their advantages and their drawbacks and therefore is selectively used for different tasks to either maximize their benefits or minimize their drawbacks. For example, “thermistors”, which are devices having a conductivity which varies with temperature, are advantageous for use in VLSI process schemes, particularly due to their simplicity of fabrication.
The effectiveness of a temperature sensor such as a thermistor is a direct function of its sensitivity. The greater the percentage of change in the conductivity of a sensor in relation to the change in temperature, the more sensitive is the sensor. Thermistors are growing in popularity due to their ease of process integration in the fabrication of ICs. In recent years there has been a continued effort to provide more sensitive thermistors. Nonetheless, the sensitivity of thermistor devices still has to be balanced with the ease of fabrication. This is especially true in thermistors that are used in conjunction with VLSI devices.
Use of thermistors in VLSI devices provides an additional advantage over the prior art sensors. In many ICs having VLSI devices, reversed-biased diodes, also known as thermal diodes, are utilized for their particular temperature sensitivity and their ease of integration into the VLSI fabrication process flow. However, recent studies have shown that when diodes are used, especially in silicon-on-insulator (SOI) or strained SOI substrates, the strained junctions cause noise and variability to the thermal diode measurement. Thermistors are then an attractive option to replace reversed-biased diodes in these circumstances. Nonetheless, any improvements to improve the temperature sensitivity of such thermistors while not affecting the ease of their fabrication process will make them even a more attractive option in such situations.
Consequently, it would be desirable to provide an integrated circuit thermistor structure.
It would further be desirable to provide an integrated circuit thermistor structure for use in conjunction with SOI and strained SOI structures in VLSI circuits.
It would further be desirable to provide methods for fabricating thermistors, which can be integrated into existing integrated circuit fabrication processes.